Not all the voice actors were from the movie ofcourse.
The problem with the current lines are not necessarily the quotes they say, but the fact that a lot of them have noises or music playing in the background, which disturbs the music or other sounds of the actual game itself. Also hearing the same quotes a thousand times, and hearing those same quotes again in the movie is quite boring and doesn't make them special anymore, don't you think?
Furthermore, judging from what I have seen in the INI files and what I have heard in-game, the amount of lines for specific situations is far less at the moment than it was in vanilla. A lot of heroes only have specific lines for walking, selecting the hero or attacking, and using these same lines for the other situations. Vanilla had specific lines for going back to the camp, retreating from battle, garissoning a tower or other building, charging (a specific line when you attack a unit for the first time), and lines for selecting him/her during the battle. At the same time the amount of lines for walking or attacking was much more than what we have now.
You would rather have Haldir calmy talk 5 minutes about his relationsip with Dwarves during a fight with orcs, than having him say to hold the lines in a way that should motivate his troops? Or when Erebor Dain says "Where is Thorin??", when Thorin is obviously dead? How is that even fitting to the game?
Last argument against this is how much size the amount of English lines take in the mod. The voice_english.big file has all the English lines for the mod, and is almost 800 mb! If we were to use the sounds from vanilla the size of the mod would decrease drastically, and if I'm not mistaken this would have impact on the performance of the game as well, meaning less lag and stuff.
I'm planning to change this for personal use anyway, so I really don't mind what we have now, but if this gets enough support, and the team is willing to change this, then I volunteer to take this task on me.